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Organic Chemistry Lab II, Spring 2009

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1 Organic Chemistry Lab II, Spring 2009
Synthesis of Alkenes 1: Miniscale Synthesis of Alkenes Via the Acid-catalyzed Dehydration of 3,3-dimethyl-2-butanol Organic Chemistry Lab II, Spring 2009 Dr. Milkevitch March 1 & 3, 2010

2 Elimination Reactions
Today: Conduct an elimination reaction One of the fundamental reactions of organic compounds What you will learn/observe: Review elimination reactions Learn how to synthesize an alkene Acid catalyzed dehydration of an alcohol Look at the products formed See which product will predominate

3 Elimination Reactions
Let’s Review: Elimination reaction: a fundamental organic reaction Two species are eliminated from a substrate Elimination mean’s they are gone, gone, gone NOT a substitution Basic mechanism:

4 Another Kind of Elimination Reaction
A dehydration reaction Multiple step reaction Loss of water, forming a carbocation Alkene formation results Mechanism: E1 mechanism E1 elimination of a protonated alcohol Reaction of alcohols Acid catalyzed Forms an alkene and water

5 General Mechanism: Acid-Catalyzed Dehydration of an Alcohol
Step 1: Protonation of hydroxyl group Step 2: Ionization to carbocation

6 Step 3: Deprotonation to give the alkene

7 Mechanism for This Reaction
Step 1: Protonation of the alcohol Step 2: Formation of the carbocation

8 Rearrangement Hydrogens available for elimination

9 Products

10 Products: Don’t Forget
Formed from secondary carbocation

11 Which Product Prevails?
More substituted alkene predominates Called Zaitsev’s Rule

12 Procedure Construct your simple distillation apparatus
Use a 25 ml RB flask Place 6 ml of 3,3-dimethyl-2-butanol into this flask Include a magnetic stir bar Add 6 ml of 85% phosphoric acid to this flask Place on apparatus Start stirring and heating Collect distillate in a graduated cylinder In an ice bath Distillate should be cloudy Don’t let the temperature rise above 75 deg C!!

13 Procedure II Complete distillation Turn off heat, let apparatus cool
Disassemble distillation apparatus Clean with acetone Set aside to dry Transfer distillate to a small separatory funnel Drain lower aqueous layer into a beaker Wash organic layer with 20 ml of saturated salt soln Drain off lower aqueous layer Pour off organic layer into a 25 ml erlenmeyer flask Dry with anhydrous magnesium sulfate

14 Procedure III Gravity filter into a clean, dry 10 ml RB flask
Reassemble distillation apparatus Distill again Cool receiving flask in an ice bath Weigh a small vial, record weight of this flask When distillation is done, transfer distillate to this vial Reweigh the distillate in the vial Do a GC analysis Determine the % of your alkenes Determine the % of your starting material (3,3-dimethyl-2-butanol) Accomplish a IR spectrum

15 Your Report Formal Report Required
Make sure to include the mechanism of the reaction in your report (separate section) Results Section: Make sure to state all results Prove that you made the alkenes Include GC chromatogram (annotated) Determine % of your alkenes IR spectrum, annotated (if completed) Conclusions section: Was the experiment successful? How much of the alkenes are produced? Any leftover 3,3-dimethyl-2-butanol? If unsuccessful, any ideas why?

16 Miniscale Apparatus for Distillation

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